Multi-layer composite membrane materials and methods therefor

ABSTRACT

Composite membrane materials and process for their production. The process entails the use of at least first and second porous membranes formed of a polymeric material and at least a third porous membrane formed of the same polymeric material, but having larger pores than the first and second porous membranes. The first, second and third porous membranes are laminated together by applying heat and pressure to the first, second and third porous membranes without applying a separate adhesive material therebetween. The laminated first, second and third porous membranes form a composite membrane in which the third porous membrane is between the first and second porous membranes.

BACKGROUND OF THE INVENTION

The present invention generally relates to composite membrane materials,and more particularly composite membrane materials formed by laminatingmultiple porous layers while maintaining a desired level of porositywithin the composite membrane material.

Porous membranes are employed in a wide variety of applications,nonlimiting examples of which are microventing, liquid filtration, andmicrofiltration systems, including water filtration systems and filterbag media for dust collectors. Desirable properties of porous membraneswill depend on the particular application, though generally controlledporosity, resistance to temperature, chemicals and/or abrasion, andwettability are often of particular interest. Porous membranes have beenproduced from various materials, including polypropylene, acrylics,polyesters, polyphenylene sulfide (PPS) such as Torcon® and Procon®,aramids such as Nomex®, polyimides such as P84, fiberglass, andpolytetrafluoroethylene (PTFE) such as Teflon®. Of these, PTFE andparticularly expanded PTFE (ePTFE) membranes have found wide use in viewof its chemical resistance and porosity characteristics. The productionof ePTFE generally entails extruding a tape formed of PTFE, and thensubjecting the tape to biaxial stretching in the plane of the tape toproduce a membrane containing pores, often micropores, i.e., pore sizesof less than one micrometer. Because PTFE is hydrophobic, treatment isrequired to allow the use of PTFE membranes for filtration applicationsin which water or a water-containing liquid is to be filtered. Varioustreatment techniques are well know for imparting hydrophobic, oleophobicand hydrophilic properties to PTFE membranes.

For certain applications, composite ePTFE membranes are constructed oftwo or more microporous ePTFE membranes that are laminated together withthe assistance of a bonding agent. Suitable bonding agents are typicallyformed of polymeric materials having lower melting temperatures thanPTFE to allow the bonding materials to melt and bond the PTFE membranestogether without melting or otherwise damaging the membranes. Thebonding agent may be applied in liquid form or as an adhesive web orfilm that is melted during the lamination process. However, the presenceof the bonding agent between PTFE membranes inevitably reduces porosityof the composite membrane, particularly if the membranes aremicroporous.

In view of the above, it would be desirable if alternative methods wereavailable by which membranes, and particularly microporous membranesformed of ePTFE, could be bonded together to form composite membranematerials.

BRIEF DESCRIPTION OF THE INVENTION

The present invention provides composite membranes and processes fortheir production. The composite membranes can be processed to besuitable for use in a wide variety of applications, including but notlimited to microventing, liquid filtration, and microfiltration systemsand processes.

According to a first aspect of the invention, a process is provided forproducing a composite membrane that entails the use of at least firstand second porous membranes formed of a polymeric material and at leasta third porous membrane formed of the same polymeric material, buthaving larger pores than the first and second porous membranes. Thefirst, second and third porous membranes are laminated together byapplying heat and pressure to the first, second and third porousmembranes without applying a separate adhesive material therebetween.The laminated first, second and third porous membranes form a compositemembrane in which the third porous membrane is between the first andsecond porous membranes.

Another aspect of the invention is a composite membrane produced by theprocess described above. According to a particular aspect of theinvention, the composite membrane comprises at least first and secondporous membranes formed of a polymeric material, and at least a thirdporous membrane between and bonded to the first and second porousmembranes without a separate adhesive material therebetween. The thirdporous membrane is formed of the same polymeric material as the firstand second porous membranes, but has larger pores than the first andsecond porous membranes. Furthermore, the first and second porousmembranes are bonded to each other by resolidified portions thereof thatextend through the third porous membrane. In a particular butnonlimiting example, the polymeric material is polytetrafluoroethyleneand the first, second and third porous membranes are expandedpolytetrafluoroethylene membranes.

In view of the above, it can be seen that a technical effect of thisinvention is that a composite membrane can be fabricated to containmultiple membrane layers formed of the same polymeric material, whichare bonded together without the inclusion of any type of extraneousadhesive material. Consequently, an extraneous bonding agent is notpresence between the individual membranes that would reduce the porosityof the composite membrane.

Other aspects and advantages of this invention will be betterappreciated from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically represents a process and apparatus for forming aporous composite membrane material comprising individual porousmembranes in accordance with an embodiment of this invention.

FIG. 2 schematically represents a cross-sectional view of a compositemembrane material of a type that can be produced by the process of FIG.1.

FIG. 3 is a microphotograph of an exemplary membrane that can be used asan intermediate membrane of the composite membrane material representedin FIG. 2.

FIGS. 4 and 5 are microphotographs of exemplary membranes that can beused as outer membranes of the composite membrane material representedin FIG. 2.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 schematically represents an apparatus 10 for carrying out aprocess for producing a porous composite membrane material 12 formed ofmultiple individual porous membranes 14, 16 and 18. An example of theporous composite membrane material 12 is schematically represented inFIG. 2, with the layers of the composite membrane material 12 shownspaced apart to facilitate an understanding of differences between theindividual porous membranes 14, 16 and 18. Therefore, it should beunderstood that FIG. 2 is drawn for the purpose of explaining theinvention and is not to scale. Furthermore, though the compositemembrane material 12 is represented in FIGS. 1 and 2 as being formed bythree individual membranes 14, 16 and 18, the membrane material 12 couldcomprise more than three individual membranes 14, 16 and 18.

The composite membrane material 12 is intended to be porous andpermeable so that a sufficiently small substance, such as a particulatesolid (for example, particles) or molecules of a fluid (including gases,vapors and liquids) can pass or flow through the membrane material 12,whereas larger solids and molecules are unable to pass through themembrane material 12. As such, the individual membranes 14, 16 and 18are also porous and permeable as a result of containing numerous pores26, 28 and 30 (FIG. 2), respectively, that extend entirely through themembranes 14, 16 and 18. Such porosity can be created by performing avariety of processes, including but not limited to perforation,stretching, expansion, bubbling, and extraction processes on nonporoussheet materials. Suitable sheet materials can be formed by a variety ofprocesses, including but not limited to extrusion, foaming, skiving andcasting. Such processes are well known, and will not be discussed in anydetail here. A preferred process for producing the membranes 14, 16 and18 is believed to be extrusion of a tape, followed by biaxial stretching(also known as tentering) to create a desired level of porosity in themembranes 14, 16 and 18. Tentering of tapes to produce porous membranesare also well known, and therefore will not be discussed here in anydetail. FIG. 3 is a microphotograph of an example of the membrane 18 andFIGS. 4 and 5 are microphotographs of examples of membranes 14 and 16that were produced by tentering during investigations leading to thepresent invention.

The individual membranes 14, 16 and 18 can be formed from variousmaterials. However, a preferred aspect of the invention is that allthree membranes 14, 16 and 18 are formed of the same polymeric material.While various candidate materials are well known, including but notlimited to polypropylene, acrylics, polyesters, polyphenylene sulfide(PPS), aramids, polyimides, and fiberglass, a preferred material for themembranes 14, 16 and 18 is polytetrafluoroethylene (PTFE), whoseexpansion yields what is known as expanded PTFE (ePTFE). Notableexamples of PTFE include the Teflon® family of resins commerciallyavailable from E. I. du Pont de Nemours & Company. It is foreseeablethat the membranes 14, 16 and 18 could be formed to contain one or moreadditives to modify certain properties, for example, antimicrobial,abrasion resistance, tensile strength, etc., resulting in properties forthe composite membrane material 12 being enhanced while retaining thebase composition of each membrane 14, 16 and 18.

For the production of the ePTFE membranes 14, 16 and 18, a suitable butnonlimiting process is to mix a PTFE resin with a lubricating agent,forming the mixture into billets, extruding the billets to form tapes,which may undergo calendering to promote the uniformity of the tapesprior to being biaxially stretched to form the membranes 14, 16 and 18.During the expansion process, the tapes are stretched (strained) beyondthe elastic limit of their material to introduce a permanent set orelongation, resulting in each membrane 14, 16 and 18 generally having amacrostructure (architecture) comprising a three-dimensional matrix orlattice-type structure in which individual fibrils are interconnected bynodes, with irregular-shaped interconnected pores 26, 28 or 30 definedtherebetween. The typical size of the pores 26, 28 and 30 will dependenton the physical and chemical properties of the membrane material and theparameters of the expansion process, though pore sizes of up to about 20micrometers are readily achievable for ePTFE membranes, with pore sizesof less than ten micrometers being more typical, and pore sizes of lessthan five micrometers being typical for microfiltration andnanofiltration processes. Thereafter, the membranes 14, 16 and 18 mayundergo sintering at temperatures below their melting temperatures fordurations that are capable of reducing stresses (anneal) and stabilizingtheir microstructures. As noted above, all of these processes are wellknown in the art and therefore will not be discussed in any furtherdetail. The final thicknesses of the membranes 14, 16 and 18 can andwill vary depending on the particular intended application for thecomposite membrane material 12. For applications in which microporosityof the membrane material 12 is desired, typical thicknesses for theindividual membranes 14, 16 and 18 may be in a range of about 2.5 toabout 250 micrometers, and typical thicknesses for the resultingcomposite membrane material 12 may be in a range of up to about 0.5millimeters. Furthermore, porosity levels of about 1% to about 97% aretypically desirable for applications in which microporosity of themembrane material 12 is desired, though lesser and greater porositylevels are foreseeable.

According to a preferred aspect of the invention, the composite membranematerial 12 is produced without the use of any extraneous adhesivematerial being used to bond the membranes 14, 16 and 18 together.Instead, as schematically represented in FIG. 2 and evident fromcomparing FIGS. 3, 4 and 5, the intermediate membrane 18 is produced tohave larger pores 30 than the two outer membranes 14 and 16, and is thenused to directly bond the membranes 14 and 16 to each other without anyintervening substance within the interface or interstitial regions 24therebetween (FIG. 2), in other words, with no substantial amount oronly a trace amount of an additional substance that is different fromthe material of the membranes 14, 16 and 18 and capable of bonding themembranes 14, 16 and 18 together. Consequently, a composite membranematerial 12 formed of ePTFE membranes 14, 16 and 18 can consistessentially or entirely of PTFE.

Bonding is preferably achieved through a thermal lamination process orsome other type of process during which the membranes 14, 16 and 18 aresubjected to pressure and heated to the extent that incipient meltingoccurs, resulting in the membranes 14, 16 and 18 being bonded togetherafter cooling with only the material of the membranes 14, 16 and 18.Such a process is schematically represented in FIG. 1, which shows themembranes 14, 16 and 18 passing between two heated rollers 20, and thensubsequently around a winder 22. While FIG. 1 represents all threemembranes 14, 16 and 18 as being simultaneously laminated together, theinvention also encompasses other lamination techniques. As an example,the intermediate membrane 18 can be initially laminated to only one ofthe membranes 14 or 16, after which a second lamination step isperformed in which the remaining membrane 14 or 16 is laminated to thesurface of the resulting lamination defined by the intermediate membrane18. The second lamination step can be performed immediately after thefirst lamination step as part of a continuous lamination process, or canbe performed later as part of an entirely separate lamination process.In certain embodiments of the invention, the membranes 14 and 16 may beidentical or otherwise interchangeable within the composite membranematerial 12, such that the lamination order is not critical.

The pressure applied by the lamination rollers 20 (as well as anyadditional rollers that may be used to apply pressure during asubsequent lamination step) should be sufficient to achieve intimatecontact between the membranes 14, 16 and 18 that will result in bondingat the elevated temperature. In lamination processes of the typerepresented in FIG. 1, the membranes 14, 16 and 18 are preferably heatedto a temperature slightly below, for example, within 20° C. of, themelting temperature of the material of the membranes 14, 16 and 18. Inthe case where the membranes 14, 16 and 18 are formed of PTFE (meltingpoint of 341° C. to about 348° C.), the membranes 14 and 16 and 18 arepreferably heated to a temperature of about 332° C. to about 340° C.,for example, about 336° C.

The larger pore size of the intermediate membrane 18 is believed to beimportant in order to maintain the porosity of the other membranes 14and 16 and, consequently, the desired porosity of the composite membranematerial 12. In particular, attempts to directly laminate ePTFEmembranes (such as membranes 14 and 16) having pore sizes and porositiesdesired for a composite membrane material have required the use oftemperatures and/or pressures that produce composite membrane materialsthat may be essentially impermeable as a result of pore blockage, forexample, due to pore distribution and/or damage to the surfaces of themembranes. In contrast, bonding of identical membranes 14 and 16 with amembrane 18 of the same material but having larger pores 30 has beensurprisingly shown to achieve acceptable bonding at temperatures andpressures that cause very little and often negligible pore blockage. Themembrane 18 can have an average pore size of at least five times greater(as schematically represented in FIG. 2), more preferably about tentimes greater, than the membranes 14 and 16. Notably, by combining thethree (or more) membranes 14, 16 and 18, the resulting compositemembrane material 12 has an effective pore size of less than theindividual membranes 14, 16 and 18 as a result of the pores of theoverlapping layers not being aligned with each other. For example,laminating membranes 14 and 16 with pore sizes exceeding one micrometerwith a membrane 18 having a pore size of five times or more greater canproduce a composite membrane material 12 that exhibits an airpermeability that is lower than a single sheet membrane having a poresize of about 0.05 micrometer. For example, membranes 14 and 16 havingpore sizes of about one micrometer have produced composite membranematerials 12 that have air permeabilities of about 10 liters/m²s, whichis comparable to prior art single sheet membranes having a pore size ofabout 0.05 micrometer. Air permeabilities of less than 10 liters/m²s,for example, about 5 liters/m²s, have also been produced. Thesecomposite membrane materials 12 also exhibited desirable characteristicsas quantified by oil rating (AATCC Test Method 118-1983), bubble pointpressure (ASTM F316), and water entry pressure or WEP (ASTM D751). Thesecomposite membrane materials 12 had oil ratings of #7 or #8, isopropylalcohol (IPA) bubble point pressures of greater than 40 psi to about 47psi (about 2.8 to about 3.2 bar), and WEP levels of about 120 to about180 psi (about 8.3 to about 12.4 bar). The multilayer construction ofthe membrane materials 12 was concluded to have promoted theseperformance factors, which are extremely important for various membraneapplications.

The larger pore size of the membrane 18 can be achieved during theexpansion process by subjecting its precursor tape to greater expansionthan the tapes used to produce the membranes 14 and 16. The resultingmembranes 14 and 16 may be essentially identical, and tend to be moreamorphous (less crystalline) than the membrane 18 as a result of thegreater extent to which the membrane 18 is expanded. Consequently,portions of the membranes 14 and 16 are able to start melting prior toany melting of the membrane 18. These molten portions of the membranes14 and 16 flow into and may flow through the pores of the membrane 18,preferably to the extent that the molten portions are able to commingleor merge together within the pores of the membrane 18. Uponresolidification of their molten portions, the membranes 14 and 16 arebonded to the membrane 18 and preferably bonded to each other by theresolidified portions 32, forming a strong bond created by aninterlocking network of resolidified portions 32 that preferably extendsentirely through the thickness of the membrane 18, as schematicallyrepresented in FIG. 2. As such, bonding of the membranes 14, 16 and 18entails incipient melting of the membranes 14 and 16, and does notrequire melting of the membrane 18 during the lamination process. Insome cases no melting of the membrane 18 will occur.

Prior to or after the lamination process, the membranes 14, 16 and 18and/or the resulting composite membrane material 12 can undergo varioustreatments, for example, a treatment that will render a compositemembrane material 12 formed of ePTFE membranes 14, 16 and 18 to behydrophilic and/or oleophobic. Such treatments are well known,nonlimiting examples of which include impregnation using atetrafluoroethylene/vinyl alcohol copolymer, coating the membraneinterior with a mixture of a fluoroaliphatic surfactant and ahydrophilic but water-insoluble polyurethane, irradiation treatment,treatment with a hydrophilic precursor acrylate terpolymer, etc.Notably, the individual membranes 14, 16 and 18 can have sufficientlylarge pore sizes (for example, about 0.1 micrometer or more) to promotepenetration of the treatment, yet yield a membrane material 12 whoseeffective pore size might otherwise limit penetration.

Following or as a result of the lamination process, the compositemembrane material 12 may be bonded to one or more additional layers thatform a substrate or backer media for the material 12, for example, as astructural element for use in microfiltration applications. Substrateand backer media are well known in the art, with nonlimiting examplesincluding one or more layers of polyester, polypropylene, polyamide,polyethylene, polyphenylene sulfide (PPS), and nonexpanded PTFE. Thesubstrate or backer media can have a variety of architectures, examplesof which include woven, scrim, nonwoven and felt.

Composite membrane materials 12 produced in the manner described abovecan find use in a wide variety of applications, with their suitabilitybeing larger dependent on the material and pore sizes of the membranes14, 16 and 18 and the effective pore size and porosity of the material12. Nonlimiting examples include microventing, microfiltration, liquidfiltration (including water purification) and hot gas filtrationprocesses performed in medical, industrial, power generation, andautomotive applications. Particular examples include the decontaminationof chemical and/or biological agents. Composite membrane materialsformed of ePTFE are particularly well suited for medical applications inwhich gamma radiation stability, sterilization including EtO (ethyleneoxide) gas sterilization, etc., are notable requirements.

While the invention has been described in terms of preferredembodiments, it is apparent that other forms could be adopted by oneskilled in the art. Therefore, the scope of the invention is to belimited only by the following claims.

The invention claimed is:
 1. A process for producing a compositemembrane material, the process comprising: processing a polymericmaterial to produce at least first and second porous membranes formed ofthe polymeric material, each of the first and second porous membranescomprising pores; providing at least a third porous membrane formed ofthe polymeric material, the third porous membrane having pores that havean average pore size greater than average pore sizes of the pores ofeach of the first and second porous membranes, and wherein the first andsecond porous membranes are more amorphous and less crystalline than thethird porous membrane; without applying a separate adhesive materialtherebetween, laminating the first, second and third porous membranestogether by applying heat and pressure to the first, second and thirdporous membranes to cause incipient melting of at least the first andsecond porous membranes, wherein during the laminating step the firstand second porous membranes start to melt prior to any melting of thethird porous membrane; and then cooling the first, second and thirdporous membranes to cause the first and second porous membranes to bondto each other by resolidified portions thereof that extend through thethird porous membrane, the first, second and third porous membranesbeing laminated together to form a composite membrane material whereinthe third porous membrane is between the first and second porousmembranes, the composite membrane material having an effective pore sizeof less than the average pores sizes of each of the first, second andthird porous membranes.
 2. The process according to claim 1, wherein thelaminating step comprises simultaneously laminating the first, secondand third porous membranes together.
 3. The process according to claim1, wherein the laminating step comprises laminating the first and thirdporous membranes together, and then laminating the second porousmembrane to the third porous membrane.
 4. The process according to claim1, wherein the processing that produces the first and second porousmembranes comprises expansion of nonporous sheets formed of thepolymeric material, and the average pore size of the third porousmembrane is produced by subjecting the third porous membrane to greaterexpansion than the first and second porous membranes.
 5. The processaccording to claim 4, wherein the expansion of the first, second andthird porous membranes causes the first and second porous membranes tobe more amorphous and less crystalline than the third porous membrane.6. The process according to claim 1, wherein the average pore size ofthe third porous membrane is least five times greater than the averagepore sizes of the pores of each of the first and second porousmembranes.
 7. The process according to claim 1, further comprisingbonding the composite membrane material to a substrate that structurallysupports the composite membrane material.
 8. The process according toclaim 1, wherein the polymeric material is polytetrafluoroethylene. 9.The process according to claim 8, wherein the first, second and thirdporous membranes are expanded polytetrafluoroethylene membranes, and theprocess further comprises forming the first, second and third porousmembranes by biaxially stretching polytetrafluoroethylene sheets. 10.The process according to claim 8, further comprising treating the first,second and third porous membranes to be hydrophilic and/or oleophobicprior to the laminating step or treating the composite membrane materialto be hydrophilic and/or oleophobic after the laminating step.
 11. Theprocess according to claim 8, wherein the laminating step comprisesheating the first, second and third porous membranes to a temperaturebelow the melting temperature of polytetrafluoroethylene butsufficiently high to cause incipient melting of the first and secondporous membranes.
 12. The process according to claim 11, wherein thebiaxially stretching of the first, second and third porous membranescauses the first and second porous membranes to be more amorphous andless crystalline than the third porous membrane.
 13. The processaccording to claim 11, wherein the average pore size of the third porousmembrane is least five times greater than the average pore sizes of thepores of each of the first and second porous membranes.
 14. The processaccording to claim 8, further comprising using the composite membranematerial in a step chosen from the group consisting of microventing,microfiltration, and liquid filtration.